I had not thought about concrete, I chose the playsand as after a little test I found that the same volume,compacted was heavier than normal sand, I put water in to fill the void within the sand and just to add further weight.. Would the oil be absorbed as well as water ?.
Currently we use 3 engineering bricks per buoy which actually works very well, these new weights have been made which would be easier to transport, not as rough on the kayak.
Looking at the link from DG I get the general impression that the rusting process will be greatly reduced as long as fresh oxygen is not added.